Conversational video phone

ABSTRACT

An improved video phone which transmits a source image over a standard telephone line in conjunction with audio voice signals is disclosed. The unique initialization protocol allows asynchronous transmission with its short format. The protocol format provides for a hardware preamble for synchronizing a receiver clock, a software preamble for identifying the video image and disabling the audio, automatic gain control adjustment data and amplitude calibration data. The audio portion is disabled by coupling a capacitor across the telephone to effectively short the telephone. The telephone is placed across the bridge circuit which is provided with switches to balance the bridge in both the telephone audio and the video modes. The transmitted as well as the received signal are passed through the same delay equalization circuit, so that half the delay equalization is done before transmission, with the remaining half being done after reception. The use of a multiplexer and demultiplexer allows the same circuit to be used. The digital signals are level shifted from 5 volts to 10 volts prior to being converted into analog, and then are level shifted down again to reduce the effects of DC offset. The digital and analog voltage supplies are coupled together with a resistor and diode, and a capacitor is coupled between the analog supply and ground to combine the reset and voltage supply functions. An RC circuit providing 180° of phase shift is coupled to the crystal oscillator to produce a clock which is locked into the series resonant frequency of the oscillator. The camera and the rest of the circuitry is turned on and off by a switch coupled to panel which can be slid in front of the camera, thus providing for the automatic protection of the camera lens when the system is switched off.

This is a division of application Ser. No. 092,346, now U.S. Pat. No.4,932,047, filed Sept. 2, 1987, which is a continuation-in-part ofapplication Ser. No. 795,967, now U.S. Pat. No. 4,715,059, filed Nov. 7,1985.

BACKGROUND

This invention relates to a video phone for transmission of audiosignals and freeze-frame video images over voice grade telephone lines.

Video teleconferencing systems typically require dedicated communicationchannels with a high bandwidth in order to transmit the large amount ofdata required to represent a complex video picture as in the case oftext or graphics. Such devices require a communication channel having abandwidth in the megahertz (MHz) range. See, for example, U.S. Pat. No.3,982,063.

Video transmission systems intended to operate over voice gradetelephone lines must contend with the fact that the usable telephonebandwidth is only approximately three kilohertz. Because of thisbandwidth limitation and a corresponding limitation in the amount ofdata that can be sent, some of these systems only send a single videoframe, rather than a moving picture. In addition, a long time isrequired for transmission, usually in the range of thirty to sixtyseconds or more. See, for example, U.S. Pat. No. 3,842,199, whichdiscloses a video (only) transmission system and U.S. Pat. No. 4,099,202which discloses a method for multiplexing a slow scan TV signal with avoice signal.

When single frames of the video image are sent, the message header canbecome a significant portion of the transmitted information.Accordingly, the protocol used for the header is important. For anasynchronous system, a new header must be sent with each data package.Accordingly, the long headers used for synchronous systems noticeablylengthen the transmission time for asynchronous, single frame systems.

Freeze-frame video systems have other problems not associated with othertelephone equipment. For instance, when a modem picks up a telephoneline, there is typically a loud click noise due to the instantaneousapplication of a voltage across the line. This is not a problem formodems, since the phone is typically not at the user's ear when themodem is connected. However, for video phones, the telephone typicallyis at the user's ear when a picture is sent.

Freeze-frame video telephones have other problems due to the necessityfor transmitting the pixel data in as short a time as possible, thuslimiting the amount of error checking, etc. that can be used withoutlengthening the transmission time. For instance, DC voltage offsets canbe picked up in the analog portion of the modem which are thentransferred over to the digital portion when the pixels are decoded,introducing errors.

In another aspect of video phones common to modems, equalization is usedat the receiver end to compensate for group delay of transmissions overthe telephone line. This typically takes the form of four or fivesections of all-pass filters with phase delay which is the inverse ofthe phase delay over the telephone line.

SUMMARY OF THE INVENTION

The present invention is a conversational freeze-frame video phone whichtransmits a source image over a standard telephone line in conjunctionwith audio voice signals. A camera is used to capture an image to betransmitted and the image is mapped onto an image field. This imagefield is displayed on a video screen. Digitized data representing thepixels is asynchronously transmitted on a modulated signal in a singleburst. Corresponding remote images are asynchronously received,demodulated, and displayed.

The asynchronous burst of video information used by the presentinvention eliminates the need for continual synchronizing as requiredfor a multiplexed system. A time-multiplexed system which transmitsportions of a picture multiplexed with voice signals requires continualsynchronization or a new header with each transmission and thus a longeroverall transmission time. The invention utilizes an initializationprotocol which signals that a video image transmission is coming. Upondetection of this portion of the protocol, audio transmission isinhibited. The remaining portion of the protocol provides for automaticgain control (AGC) and amplitude calibration prior to reception of thevideo image data itself.

The system of the present invention may incorporate its own telephone orhave a connector for connection to the user's existing telephone. Atelephone line interface circuit is provided for coupling either thetelephone or the video picture modem to the telephone line withoutintroducing a clicking noise by the instantaneous application of thevoltage across the telephone line. This is accomplished by adding somecircuitry to a hybrid bridge circuit. A pair of switches are providedfor disabling the phone by coupling a capacitance across the phone andfor switching an RC circuit across one bridge leg and a resistor acrossthe second bridge leg to balance the bridge with the telephone removed.A resistor is coupled to the capacitor used to disable the phone so thatthe capacitor is charged to almost its full value prior to the time thephone is disabled. This resistor is then shorted out by one of theseswitches, but no large voltage is applied because the capacitor isalready substantially charged.

The present invention includes a hybrid bridge that joins a commontelephone, a video unit, and a telephone line. This hybrid bridge isunique because it accomplishes several things in a simple circuit. Thesound picked up by the telephone microphone is virtually kept frominterfering with transmitting and receiving pictures. In the picturemode, the signals generated by sound are attenuated by connecting alarge capacitor across the telephone during this time. These signals arealso attenuated for the picture receiver circuit because the circuitdoubles as a hybrid bridge.

When in telephone mode, the video unit must detect a preamble thatstarts the process of receiving a picture. It is important to reducesignals from the telephone microphone that go into the picture receivercircuit. Since the telephone must be active, a way to get attenuation isfor the circuit to double as a hybrid bridge in telephone mode too.

In both cases, the hybrid bridge is designed to give cancellation to thetelephone microphone signal going into the picture receiver circuit.

The present invention uses a unique delay equalization circuit, with thesame circuit being used on both the transmitting and receiving ends.Thus, the transmitted signal is provided with a phase delay to offset aportion of the anticipated telephone line phase delay, with theremaining compensation phase delay being provided on the receiving end.A pair of multiplexers are provided to route the signal through the samedelay equalization circuit on both the transmitting and receiving ends.

A feedback circuit is provided for diagnostic purposes to allow atransmitted signal to be routed back through to the receiver withoutbeing sent over the telephone line. This is accomplished by appropriatecontrol signals to the multiplexers coupled to the delay equalizationcircuit.

The present invention also provides combined analog voltage supply andreset circuit. In a typical prior art video phone, which incorporatesboth analog and digital circuitry, the digital power supply is turned onand a separate RC delay circuit provides a reset delay before thevoltage is applied to the analog circuitry. The present inventioncouples the digital voltage source to the analog voltage source througha resistor and a diode. The analog voltage source is then coupled toground through a capacitor. The capacitor quickly discharges when poweris turned off, and then turns on slowly to provide a reset delay whenpower is applied.

The camera of the present invention is provided with a panel which isslid across the front of the camera lens to block the camera when not inuse. This protects the camera from light burns and the camera tube fromambient light, while also protecting the camera lens from dustcontamination and providing privacy to the user. This panel is coupledto the on/off switch for the circuitry of the present invention, thusforcing the user to protect the camera when the device is turned off.

An improved crystal oscillator is used in the present invention. Thecircuit is designed to be a phase-shift RC oscillator even by itself,without the crystal. The circuit is designed to oscillate at the crystalfrequency even if a capacitor were substituted for the crystal. Thecapacitor would be large enough to be considered virtually a zeroimpedance so that it would not influence the oscillator frequency.

Since the crystal exhibits it's lowest impedance, and zero phase shift,at it's series resonance frequency, the oscillator will function thesame when the capacitor is replaced by the crystal. The difference isthat the oscillator is now crystal controlled, with the crystaloscillating at it's series resonance frequency.

This crystal oscillator has advantages over many other crystaloscillators:

1. It won't go into an undesirable overtone mode because there is notenough loop gain at those higher frequencies for oscillation.

2. The crystal frequency is not sensitive to component values. Somecrystal oscillator circuits include an LC tank circuit to guarantee thatthe oscillator won't oscillate at an overtone frequency. The problemwith this is that the inductor and capacitor have a strong influenceover the crystal frequency. This sensitivity forces the designer to usean expensive inductor and capacitor with tighter percent tolerances.

The present invention also provides analog circuitry for performing themodem functions and the transmitting and receiving. Digital-to-analogand analog-to-digital converters are provided for the input and outputof this analog circuit. In order to reduce the effects of DC voltageoffset, the analog signals are processed at higher voltages. The digitalsignals are level shifted up (i.e., from 5 volts to 10 volts) at theirinput to provide higher level signals for switching the higher levelanalog signals. The signals are then processed, and when the analogsignal is converted into digital form, the digital signal is levelshifted down to the lower voltage level. Thus, any DC voltage offsetpicked up in the circuit will be reduced in half by the level shifting,thus improving the systems signal-to-noise ratio.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a video system for coupling to atelephone according to the present invention;

FIG. 1A is a perspective view, partially broken away, of a portion ofthe video system of FIG. 1 showing the camera protection panel;

FIG. 2 is a block diagram of the electronic circuitry of the presentinvention;

FIG. 3 is a diagram of the protocol for the header according, to thepresent invention;

FIG. 4 is a diagram of the telephone line interface circuit of thepresent invention;

FIG. 5 is a detailed circuit diagram of the circuit of FIG. 4;

FIG. 6 is a circuit diagram of an oscillator according to the presentinvention;

FIG. 7 is a circuit diagram of the combination digital and analogvoltage supply and reset circuit according to the present invention;

FIG. 8 is a block diagram of the analog circuit of the present inventionshowing the delay equalization and diagnostic feedback circuits; and

FIG. 8A is a circuit diagram of a level shifting circuit used in thecircuit of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective view of a video system 10 according to thepresent invention which can be coupled to a user telephone 12 shown inphantom. The video system includes a display screen 14 and a camera 16.A number of pushbuttons are provided. A pushbutton 18 allows the user tosend a video image. A pushbutton 20 switches the display between thereceived image and the mirror image of the user as observed by camera16. Pushbuttons 22 and 24 adjust the brightness up and down,respectively, of the image on screen 14. A privacy panel 25 can be slidin front of camera 16 to protect it when the unit is not in operation.

FIG. 1A shows privacy panel 25 in more detail. The panel slides indirections 26. When the panel passes in front of camera lens 16, anupperly protruding member 28 will trip a microswitch 30 which will turnoff power to the circuitry of the present invention. Thus, by the simpleact of turning off the video unit, a user is forced to protect thecamera lens. This protects the camera from ambient light burns when theunit is turned off.

FIG. 2 is a block diagram of the circuitry of the present invention. ATV camera 32 is used to capture a video image of the telephone user. Theimage from camera 32 is fed through an analog-to-digital (A/D) converter31 to an image control processing (ICP) circuit 34. ICP circuit 34 canbe a standard video display processor with additional circuitry fordigitizing the video image. The video display processor portion containsthe video sync generator. Processing circuit 34 can also retrieve thedata from RAM 36 and convert the data into analog form by means of adigital-to-analog (D/A) converter in circuit 34. The analog values arethen fed to a cathode ray tube (CRT) display 38 to display the image.Processing circuit 34 also transfers data from RAM 36 to amicroprocessor bus 40. A modem is provided from analog circuit 42,demodulator 41 and A/D converter 43. A preferred embodiment of the modemis disclosed in co-pending application Ser. No. 06/725,091. The modemoperates to modulate a carrier signal with the image data and feed thesignal through a telephone line interface circuit 44 for transmission tothe telephone line.

The operation of the video phone is controlled by a microprocessor 56which includes RAM and ROM. Microprocessor 56 is powered by a powersupply 60 which is fed by the 115 volt AC line voltage.

A voltage reference and reset circuit 70 is used to provide analogpower.

The protocol used by the video phone to initiate an interruption ofaudio transmission for the transmission of a video image asynchronouslyestablishes a communication link in a short period of time. Due to theneed for keeping interruptions of audio transmissions short, the longprotocols used for TV or facsimile machines are not adequate for thispurpose. As shown in FIG. 3, the protocol is initiated by a hardwarepreamble 72 which synchronizes a phase lock loop in the demodulator ofthe receiving video phone. This pattern is detected by the modemhardware in the receiver.

A preferred modulation scheme using two phases and eight amplitudelevels is disclosed in co-pending patent application Ser. No.06/745,091, filed June 14, 1985, which is hereby incorporated byreference. For this modulation scheme, the hardware preamble consists ofa carrier burst at the maximum amplitude of the modulation levels at afirst phase which is transmitted sixteen times. This is followed by adata clock sync pattern which is transmitted a total of twenty times.The data clock pattern consists of a symbol at maximum amplitude at thefirst phase followed by a symbol at maximum amplitude at a second phase.A second carrier burst at maximum amplitude in the first phase is thentransmitted eight times. This is followed by a pattern transmitted fourtimes which consists of a carrier burst at maximum amplitude in thefirst phase for four symbols followed by an eight symbol pattern ofphase changes at maximum amplitude.

Upon recognition of the hardware preamble by the modem, microprocessor56 directs telephone line interconnect circuit 44 to disconnect thetelephone until the video image is received. Following the hardwarepreamble, which provides for the initial synchronization of theequipment, a software preamble 74 is sent and received and verified bythe software in the receiver to indicate that a video frame is tofollow. Following the software preamble, a pattern 76 is transmitted toprovide for automatic gain control (AGC) adjustment. After the AGCpattern, a calibration line pattern 78 is transmitted, giving a patternof symbols at each amplitude level to calibrate the receiver to theactual amplitude levels received.

Following the calibration pattern, an ID line 80 can be sent to indicatethe configuration of the video phone sending the data and the type ofvideo image being sent.

Following the protocol, the data representing the pixels of a videoimage are sent according to a modulation scheme which minimizes theeffect of errors while maximizing transmission speed. Such a modulationscheme is shown in co-pending application Ser. No. 06/745,091.

FIG. 4 shows telephone line interface circuit 44 of FIG. 2 in moredetail. A telephone 82 is coupled across nodes 84 and 86 of a bridgecircuit. The bridge circuit has two additional nodes 88 and 90. Thebridge has four legs, consisting of transmitting transformer 92, thetelephone line 94, a resistor 96 and an RC circuit consisting ofresistor 98 and capacitor 100. A receiver transformer 118 is coupledacross nodes 88 and 90 through a capacitor 120.

A pair of switches 102 and 104 are provided. These switches arecontrolled by the microprocessor to configure phone detect circuit 44 ineither the telephone mode or the modem picture mode. The switches areshown in the telephone usage mode with an RC circuit consisting of aresistor 106 and a capacitor 108 being coupled in parallel withtransmitting transformer 92 by switch 102. Switch 104 couples a resistor110 in parallel with resistor 96. These parallel combinations balancethese two legs of the bridge during telephone usage.

The position of switches 102 and 104 shown in phantom couples a circuitin the picture mode. In this mode, a capacitor 112 is coupled acrosstelephone 82 between nodes 84 and 86. This effectively shorts out thetelephone during the picture mode. A resistor 114 is a low valueresistor used as a fuse. In order to balance the bridge circuit, switch104 switches resistor 110 in parallel with resistor 98 to lower theeffective value of the resistance in that leg of the bridge.

A charging resistor 116 is coupled to capacitor 112 to insure thatcapacitor 112 will have a voltage imposed across it. Thus, when switch102 switches to the picture mode and short circuits resistor 116, thereis not a large voltage jump and a corresponding click which would beheard on the telephone.

The bridge circuit of FIG. 4 has two voltage divider legs, A and B. Ifcircuit B is properly selected, there is cancellation of voice ECT fromthe phone into the receiver circuit through receiver transformer 118.This is a balanced duplexer that tends to keep telephone pickup frominterfering with the protocol preamble. The resulting receiver gain ismade low because network B is determined by network A. Receivertransformer 118 is a 10 kilohm to 10 kilohm transformer which sees areceiver circuit load of 10 kilohms. Transmitter transformer 92 is a 600ohm to 600 ohm transformer with the transmitter having an impedance of600 ohms. This is matched to the characteristic telephone line impedanceof approximately 600 ohms.

In the picture mode (switch position in phantom) the receiver gain ismuch higher than in the case of telephone usage. Without the use ofcapacitor 112 to short out telephone 82, the circuit would be imbalancedand voice and noise pickup would tend to interfere with the picturetransmission and reception.

The circuit of FIG. 4 is shown in more detail in FIG. 5. A transmittercircuit 122 is coupled to transmitting transformer 92 and receivessignals from a microprocessor on an input line 124. A receiver circuit126 is coupled to receiving transformer 118.

Switches 102 and 104 are activated by a relay 126 through a transistor128 coupled to a control line 130. The control signal on control line130 is provided by the microprocessor.

In addition to activating relay 126, the control signal serves toactivate a transistor 132 which switches a resistor 134 to be inparallel with a resistor 136. Resistor 136 is a gain setting resistorfor an operational amplifier 138 in receiver 126. By switching resistor134 in parallel, the gain of operational amplifier 138 is lowered in thepicture mode. This is because, in the picture mode, resistor 110 shuntsresistor 98, thus lowering the attenuation produced by this resistorcombination. The gain of operational amplifier 138 is accordinglylowered to compensate.

FIG. 6 shows clock generator circuit 50 in detail. A crystal oscillator140 is coupled to an amplifier consisting of transistors 142 and 144.The output of the amplifier is coupled back to the crystal oscillatorthrough a three stage RC network 146. The resistors and capacitors of RCcircuit 146 are chosen to give 180° phase shift, which, in conjunctionwith the 180° phase shift through transistors 142 and 144 providespositive feedback in the series resonant mode of oscillator 140. Thecapacitors can have 10% tolerance since the circuit is insensitive tocomponent values. The RC circuit attenuates the harmonics of thecrystal, but gives a low pass filter at the oscillator series resonantfrequency. The circuit thus prevents the overtone mode of the crystaloscillator from occurring in which it vibrates at a harmonic frequency.

FIG. 7 shows the voltage reference for the digital and analog powersupplies in combination with a reset capability. A 5 volt digitalvoltage supply 148 is coupled to a resistor 150 and a diode 152. Theanalog 5 volt voltage supply is taken from the other end of the resistorand diode at a point 154. The analog and digital voltage supplies areisolated because the rapid movement of the digital signals producesnoise which would otherwise impair the analog circuitry's operation. Acapacitor 156 is applied to give a reset capability. When power isturned off, capacitor 156 discharges quickly. When power is reapplied,the analog voltage is delayed by the amount of time corresponding to theRC time constant of resistor 150 and capacitor 156.

FIG. 8 shows the analog circuit and modem 42 of FIG. 2 in more detail.Data from the microprocessor is provided on data lines D0-D7 to atransmitter latch 160 and a receiver latch 162. The data to betransmitted is provided from transmitter latch 160 to adigital-to-analog converter (DAC) 164. The analog output is providedthrough an amplifier 166 to a multiplexer 168. A control signal on aline 170 selects the transmit (TX) input of multiplexer 168 for apicture transmission. The signal is amplified again in an amplifier 172and supplied to a filter 174. The signal is then provided through adelay equalizer 176 to a demultiplexer 178. The transmitted signaloutput (TX) is selected by a control line 180 and provided through abuffer 182 to phone detect circuit 44 of FIG. 2 for transmission on thetelephone line.

Filter 174 and delay equalizer 176 anticipate some of the distortions tobe suffered by the signal on the telephone line and partially compensatefor these. The remaining amount of compensation is done by the receivingcircuit which would be identical to FIG. 8. The receive signal isprovided through a receiver input (RX) of multiplexer 168. This receiveinput is selected in the receive mode by a control signal on line 170.The receive signal is then passed through filter 174 and delay equalizer176. The filter and delay equalizer on this end of the transmissioncomplete the compensation for the group delay and other distortions onthe telephone line. The receive signal is then passed through thedemultiplexer 178 on the RX output under the control of a control signalon line 180.

Thus, by the use of multiplexer 168 and demultiplexer 178, thetransmitted and received signals can share the same filter and delayequalizer, thus allowing the same amount of filtering and equalizationto be done by one-half the circuitry since the transmitted signal ispassed through the filter and equalizer twice, once on the transmissionside and once on the receive side.

The received data is passed through a programmable gain control circuit182, through a capacitor 184 to asynchronous demodulator 186. The signalthen proceeds through a baseband filter 188, the output of which isprovided to a clock recovery circuit 190 for recovery of the data clock.A sample and hold circuit 192 is also coupled to the output of basebandfilter 188 to provide a digital output. The circuit is clocked by asignal at twice the carrier frequency on a control line 194.

The present invention also provides analog circuitry for performingmodem functions during transmit and receive. This analog circuitry isaccessed by the computer system by digital-to-analog converter 164 andanalog-to-digital converter 192. System performance is directlyproportional to the DC stability of the analog circuitry. Since theanalog (modem) circuitry is a sample data system (switched capacitor) itis not immune to DC offset voltages, due to charge injection from theswitched capacitor filter sections. A clever way to minimize DC offset,resulting in a system performance gain of up to 8 dB is an analog levelshift. The system's A/D converter 192 is adjusted to receive a 2 Vp-pwave centered on 2 VDC. The modem analog circuitry is centered at 5 VDCwith the capacity of a 10 Vp-p wave. The modulated data, after passingthrough Tx-Rx filter 174 and programmable gain control (PGC) 182 is ACcoupled (through capacitor 184) to demodulator 186 to establish a DCreference. After demodulation, the signal is passed into baseband filter188 (where it will parasitically pick up the DC offset).

After baseband filter 188, the signal is converted to digital form withsample and hold circuit 192 and then passed through a 2:5 resist divider(resistors 200, 202) to ground, which reduces the DC level from 5 VDC to2 VDC, consequently reducing the signal from 5 Vpp to 1 Vpp. The outputis provided on a line 204. It is also evident that any DC offset pickedup in baseband filter 188, say 50 mV, is divided by 2:5 and therebyreduced to 20 mV. This is an 8 dB improvement over taking the signal outof the baseband undivided and passing it into an A/D converterreferenced at 5 VDC.

In order to be able to handle the higher voltage analog signals, thedigital input data signals D0-D7, and the control signals, are levelshifted from 5 volts to 10 volts via a circuit such as that shown inFIG. 8A. FIG. 8A shows a current mirror with an input on a line 196which is referenced to 5 volts and an output on the line 198 which isreferenced to 10 volts.

Returning to FIG. 8, a voltage divider composed of resistors 200 and 202provides an output signal on a line 204 which is divided down to bereferenced to 5 volts. Any DC offset picked up will also be divided inhalf, thus reducing its impact on the data signal.

The circuit of FIG. 8 also allows a diagnostic feedback mode. Byappropriate control signals, a control signal can be provided through aloop back AND gate 204 to select the transmit input for multiplexer 168while the receive output of demultiplexer 178 is selected. The videounit circuitry can thus transmit to itself for diagnostic checkout ofthe various circuit elements.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. For example, otherconfigurations of impedances could be used to balance the various legsof the bridge of FIG. 4 and FIG. 5. Accordingly, the disclosure of thepreferred embodiment of the present invention is intended to beillustrative, but not limiting, of the scope of the invention which isset forth in the following claims.

What is claimed is:
 1. A method for transmitting a single frame of avideo image and audio signals over an ordinary telephone line comprisingthe steps of:asynchronously transmitting protocol data, said protocoldata including hardware preamble data for use by a remote video phone tosynchronize a clock at said remote video phone with said protocol data,software preamble data for identifying the following data asrepresenting a video image, automatic gain control adjustment data, andamplitude calibration data for correlating each received amplitudemodulation level to a corresponding assigned amplitude modulation level;and asynchronously transmitting over said telephone line a modulatedsignal representing said video image in a burst.
 2. The method of claim1 further comprising the step of disabling an audio transmitting andreceiving means at said remote video phone in response to reception ofsaid hardware preamble data.
 3. The method of claim 1 wherein saidmodulated signal comprises a plurality of phases and amplitude levelsand wherein said amplitude calibration data comprises each of saidamplitude levels at a first phase and at least one of said amplitudelevels of each of said plurality of phases.
 4. In a conversationalfreeze-frame video phone for transmitting a single frame of a videoimage over an ordinary telephone line, the improvement comprising:meansfor transmitting and receiving an audio signal; and means, coupled tosaid telephone line, for asynchronously transmitting a modulated signalrepresenting said video image in a burst, said transmitting means beingoperative to transmit protocol data prior to transmission of said burst,said protocol data including hardware preamble data for use by a remotevideo phone to synchronize a clock at said remote video phone with saidprotocol data, software preamble data for identifying the following dataas representing a video image, automatic gain control adjustment data,and amplitude calibration data for correlating each received amplitudemodulation level to a corresponding assigned amplitude modulation level.5. The apparatus of claim 4 further comprising means for disabling saidaudio transmitting and receiving means.
 6. The apparatus of claim 4wherein said modulated signal comprises a plurality of phases andamplitude levels and wherein said amplitude calibration data compriseseach of said amplitude levels at a first phase and at least one of saidamplitude levels of each of said plurality of phases.